1. Field of the Invention
The invention relates to a lignin modified, and especially a lignosulfonate-modified, phenol-formaldehyde resin useful in adhesive compositions for making boards such as plywood, particle board and the like, for use in a binder for insulation and in other molded objects.
2. Description of Related Art
The manufacture of plywood, hardboard, particleboard and the like are well known in the art. Generally these boards are produced using cellulosic materials such as wood veneer, random geometric configuration wood chips, or wood fibers. The boards may be formed of a single homogeneous ply or may be multi-ply. Generally these board products are manufactured using an adhesive which cures under the application of heat and pressure for predetermined time periods to form the finished product.
The hot press conditions used to consolidate board products may vary widely depending upon such operating factors as type of panel, thickness of panel, etc. Press temperatures often range from about 120.degree. C. to 320.degree. C. (about 250.degree. F. to 600.degree. F.), such as about 170.degree. C. (about 330.degree. F.), press forming pressures often vary from about atmospheric up to about 70 kg/cm.sup.2 (about 1000 psi), such as from about 3.5 kg/cm.sup.2 to about 14 kg/cm.sup.2 (about 50 psi to about 200 psi), and press time periods often vary from about one minute to about ten minutes.
Many types of adhesive glue systems have been used or proposed to form such products. Two well known systems are the phenol-formaldehyde resole resin system and the urea-formaldehyde resin system. Since the base resin used in these adhesive glue systems generally constitutes the major proportion of the manufacturing cost, extenders, fillers and the like have been used commercially to reduce the cost of the adhesive glue. Extenders and fillers typically are used to reduce the amount of costly resin solids in the adhesive glue. The prior art also has proposed the use of reactive extenders, such as sulfonated lignin materials (lignosulfonates), as partial replacement for the phenol or formaldehyde used in resin preparation. Unfortunately, known lignin-based extenders and fillers have not been highly successful due to the fact that the resulting strengths of the board products using such materials as well as other board properties, are generally impaired.
For a satisfactory performance, adhesive glues for making plywood must satisfy certain working characteristics under modern production methods in addition to having the appropriate cured adhesive properties. Thus, even though good bond strengths may be possible under certain conditions with particular resin adhesives, the resin adhesives still may not be suitable for use in an adhesive glue for bonding wood composites if the adhesive working characteristics are such that good bond strengths cannot be readily achieved under accepted methods employed in the application. Therefore, there remains a need in the art for an economically attractive lignin-modified resin useful in an adhesive glue having good strength characteristics, a sufficient latitude in working properties and the ability to maintain its strength when wet.
The use of lignin-modified, and particularly lignosulfonate-modified, phenol-formaldehyde resole resins in adhesive compositions for bonding sheets of wood together is known. Lignosulfonate-modified phenol-formaldehyde resins also have found use as fluid control agents. U.S. Pat. No. 3,095,392 to Herrick, discloses a method of making phenol-formaldehyde resins modified with lignosulfonate prepared at a formaldehyde to phenol mole ratio in the range of from 2.0 to 3.0. According to this technology, a methylolphenol resin first is prepared by reacting phenol and formaldehyde at the above-mentioned formaldehyde to phenol mole ratio, and then the resin is further condensed with lignosulfonate under an alkaline condition to produce the modified resin.
U.S. Pat. No. 3,658,638 to Ludwig, discloses the preparation and use of a lignosulfonate-modified phenol-formaldehyde resin as an adhesive in the manufacture of plywood and particleboard. According to Ludwig, an alkali metal lignosulfonate and a phenol first are pre-reacted under an alkaline condition, and then the lignosulfonate-phenol adduct is reacted with formaldehyde in the presence of an alkali metal hydroxide to condense the lignosulfonate-phenol product with formaldehyde. Ludwig indicates that up to about half of the phenol normally used to prepare a resole resin may be substituted with lignosulfonate.
U.S. Pat. No. 3,931,072 to Coyle, discloses a method of making an improved adhesive resin composition in which lignin sulfonate first is reacted with formaldehyde under the influence of caustic and heat and then the resulting product is reacted with a phenol-formaldehyde resole resin previously prepared using standard procedures. U.S. Pat. No. 4,105,606 to Forss, discloses a method of making an adhesive resin composition comprising adding either alkali lignin or lignosulfonate and formaldehyde together to a preformed phenol-formaldehyde resole resin. U.S. Pat. No. 4,303,562 to Hollis, discloses a method of making an adhesive resin composition wherein a lignin-phenol concentrate, formaldehyde and sodium hydroxide are added to a preformed phenol-formaldehyde resin. U.S. Pat. No. 4,701,383 to Janiga, discloses a method of manufacturing a lignosulfonatephenol-formaldehyde resin by mixing lignosulfonate with phenol before any substantial condensation reaction between phenol and formaldehyde.